1. Field of the Invention
The invention relates to slow wave structures and more particularly to slow wave structures which are used as feedback elements in oscillator-type circuits.
2. Background of the Invention
The usefulness of RF or microwave means for monitoring liquid level is recognized by the prior art. Such devices can operate with either RF or microwave excitation. When an electromagnetic field is excited in a container partially filled with liquid, parameters of the electromagnetic field, such as resonant frequency, vary with the level of the liquid. In particular, the state of the art is shown in V. A. Viktorov xe2x80x9cMicrowave Method of Level Measurementxe2x80x9d, The Resonance Method of the Level Measurement, Moscow: Energija. 1987, disclosing an electrodynamic element, made as a section of a long line, inserted into a monitored container where the resonant frequency is measured.
A general discussion, see Viktorov V. A., Lunkin B. V., Sovlukov A. S. xe2x80x9cMethod of and Apparatus for Level Measurement by Hybrid Electromagnetic Oscillation Excitationxe2x80x9d, Radio-Wave Measurements, Moscow: Energoatomizdat, 1989, states that an electrodynamic element is placed in a monitored container, and the element""s resonant frequency has a direct correlation to the level of liquid within the container.
Slowed electromagnetic waves and slow-wave structures are also well known in the field of microwave engineering, see J. R. Pierce, xe2x80x9cTraveling-Wave Tubesxe2x80x9d D. Van Nostrand Company, Inc., Princeton, N.J., 1950. These waves are electromagnetic waves propagating in one direction with a phase velocity vp that is smaller than the velocity of light c in a vacuum. The relation c/vp is named slowing or deceleration and is designated as n. In most practical cases, slowed electromagnetic waves are formed in slow-wave structures by coiling one or two conductors (for example, into a helix, as shown in FIG. 1 of U.S. patent application Ser. No. 09/134,056, filed Aug. 14, 1998 (prior art) U.S. Pat. No. 6,293,142, where the other conductor is a cylinder), which increases the path length traveled by the wave, or by successively connecting resonant elements or cells, energy exchange between which delays the phase of the wave, or by using an electrodynamically dense medium (usually a dielectric), or a combination of these methods. Additional deceleration was also obtained due to positive electric and magnetic coupling in coupled slow-wave structures, see V. V. Annenkov, Yu. N. Pchelnikov xe2x80x9cSensitive Elements Based on Slow-Wave Structuresxe2x80x9d Measurement Techniques, Vol. 38, #12, 1995, pp. 1369-1375.
Slow-wave structure-based sensitive elements are known in the art, see Yu. N. Pchelnikov, I. A. Uvarov and S. I. Ryabtsev, xe2x80x9cInstrument for detecting Bubbles in a Flowing Liquidxe2x80x9d, Measurement Techniques, Vol. 22, #5, 1995, pp.559-560, and Yu. N. Pchelnikov, xe2x80x9cPossibility of Using a Cylindrical Helix to Monitor the Continuity of Mediaxe2x80x9d, Measurement Techniques, Vol. 38, #10, 1995, pp.1182-1184. The slowing of the electromagnetic wave leads to a reduction in the resonant dimensions of the sensitive elements and this enables one, by using the advantages of electrodynamic structures, to operate at relatively low frequencies, which are more convenient for generation and are more convenient for primary conversion of the information signal, but sufficiently large to provide high accuracy and high speed of response. The low electromagnetic losses at relatively low frequencies (a few to tens of megahertz) also helps to increase the accuracy and sensitivity of the measurements. The slowing of the electromagnetic wave leads also to energy concentration in the transverse and longitudinal directions, that results in an increase in sensitivity, proportional to the slowing factor n. See V. V. Annenkov, Yu. N. Pchelnikov xe2x80x9cSensitive Elements Based on Slow-Wave Structuresxe2x80x9d Measurement Techniques, Vol. 38, #12, 1995, pp. 1369-1375.
Most slow-wave structures were made as two-conductor periodic transmission lines (see Dean A. Watkins xe2x80x9cTopics in Electromagnetic Theoryxe2x80x9d, John Willy and Sons, Inc. Publishers). It is also possible to design a slow-wave structure which contains three or more different conductors. In all cases, the slowed wave is excited in the electrodynamic element between different combinations of the two conductors. The coiled conductors increasing the wave path are named xe2x80x9cimpedance conductorsxe2x80x9d, and conductors with simple configuration such as rods, tapes, etc., stretched along the wave propagation direction are named xe2x80x9cscreen conductorsxe2x80x9d, see V. V. Annenkov, Yu. N. Pchelnikov xe2x80x9cSensitive Elements Based on Slow-Wave Structuresxe2x80x9d Measurement Techniques, Vol. 38, #12, 1995, pp. 1369-1375.
Both the prior art and the present invention measure one or more parameters of an electromagnetic field. Some of the prior art methods and the present invention use an electrodynamic element, some are made as a resonant cavity filled with the liquid to be measured or made as an electrodynamic element placed in or outside a container. The electrodynamic element is connected to an external RF or microwave signal generator which is used to excite an electromagnetic field. The change in, for example, the level of the liquid, causes a shift in the characteristics of the electromagnetic field in the electrodynamic element. The shift in characteristics correlates to a change, for example, in the level of the liquid within the measured container.
Devices used in the prior art exhibit several problems which are overcome by the present invention. Previous methods depend upon the sensitivity of a measured parameter of an electromagnetic field to measure level displacement and provide signal resolution. Sensitivity and resolution increase with frequency. However, the increase in frequency is accompanied by an increase in electromagnetic losses, such losses causing a loss of accuracy of the measurement. Besides, it is known that the higher the frequency, the higher is the cost of electronics. The relatively low accuracy realized from the prior art is also due to resonant frequency dependence on the monitored liquid""s electric parameters. Thus, there is a need in the art for an electromagnetic method and apparatus for monitoring liquid levels and other height measurements that has better sensitivity, better resolution, greater diversity and lower cost.
Accordingly, slow wave structures have been used as feedback elements in oscillator-type circuits in which the slow wave structure acts as a delay line or as a phase-shifting element. See V. V. Annenkov, Yu. N. Pchelnikov xe2x80x9cSensitive Elements Based on Slow-Wave Structuresxe2x80x9d Measurement Techniques, Vol. 38, #12, 1995, pp. 1369-1375. This paper also shows sensors which have been developed based on making sensitive elements which are designed as sections of a slow wave structure.
In prior art slow wave sensors, an oscillator frequency changes in response to a change in phase shift and/or change in delay time of the sensitive element of the slow wave structure. These changes are proportional in response to changes in the parameter being sensed, thereby allowing an output to be derived from the sensed parameter. The oscillator frequency changes plus or minus a percentage of a nominal frequency. However, these circuits require a nominal frequency which is difficult to change to a current or voltage output proportional to the change in frequency because of the high frequencies involved. For example, in the prior art slow wave structure sensor systems, the oscillating frequency would have a nominal frequency of ten megahertz and have a variation due to the sensed parameter such as plus or minus one megahertz. The resulting nine to eleven megahertz signal must then be conditioned into representative engineering units by additional hardware and/or software. This would, for example with hardware, include a frequency-to-voltage converter, a voltage-shifting circuit, and a variable gain amplifier. The analog voltage shifting circuit must be. carefully designed to avoid temperature sensitivity and maintain stability because the zero shift is usually a factor of several times greater than the signal level, e.g. nine megahertz plus or minus two megahertz.
It is an object of the present invention to avoid many of the elements required to make this conversion.
A sensor is disclosed having a slow wave structure used with an oscillating circuit and at a frequency determined by the associated slow wave structure sensitive element. The slow wave structure is positioned such that the oscillation frequency varies with the sensed parameter. A reference frequency is subtracted from the oscillator frequency, resulting in a frequency variation for the sensed parameter of near zero Hz to a full scale of one to two MHz. The zero frequency is accomplished through the use of a local oscillator and mixer circuit to subtract the offset frequency required by the oscillator circuit, such as ten megahertz. Thus, the resulting signal frequencies start from zero or near zero. Accordingly, the frequency would vary between zero and the range of the signal frequency, such as for example, two megahertz. A frequency-to-voltage converter could be easily used and scaled to provide a zero-to-five or zero-to-ten volt output proportional to that frequency. Alternatively, an offset such as 0.50 megahertz may be used so that the range would be 0.50 to 2.50 megahertz corresponding to the use of a frequency-to-current converter directly scalable to 4 to 20 milliamps. This permits the local oscillator to have a fixed frequency. Therefore it may be crystal controlled or ceramic filter controlled, resulting in a very stable frequency.